1. Field of the Invention
The present invention relates to brightness control of a video display appliance, and more particularly to a brightness control apparatus for a video display appliance which can keep the brightness of a picture uniformly over the whole screen and thus provide good picture quality.
2. Description of the Related Art
FIG. 1 is a block diagram illustrating the construction of a conventional brightness control apparatus for a video display appliance. Referring to FIG. 1, the conventional brightness control apparatus includes a video amplifying section 1 for amplifying an input RGB (red, green, blue) video signal, a variable resistor VR1 for adjusting the amplification rate of the video amplifying section 1, and a CRT (cathode ray tube) 2 for displaying on its screen the video signal amplified by the amplifying section 1.
The operation of the conventional brightness control apparatus as constructed above will be explained.
When the RGB video signal is inputted to the video amplifying section 1, it is amplified with a predetermined amplification rate by the video amplifying section 1, and then the amplified video signal is outputted to the CRT 2, so that the video signal is displayed on the screen of the CRT 2.
The amplification rate of the video amplifying section 1 is determined by varying the resistance value of the variable resistor VR1. Specifically, as the resistance value of the variable resistor VR1 varies, the voltage supplied from the variable resistor VR1 to the video amplifying section 1 varies, resulting in that the amplification rate of the video amplifying section 1 also varies, and this causes the brightness of the video signal being displayed on the screen of the CRT 2 to be adjusted accordingly.
FIG. 2 is a view illustrating the brightness state of the picture displayed by the conventional brightness control apparatus. As described above, the conventional brightness control apparatus amplifies the input video signal with a predetermined amplification rate and outputs the amplified video signal to the CRT 2. At this time, a user can adjust the brightness of the picture by varying the amplification rate of the video amplifying section 1 by means of the variable resistor VR1.
According to the conventional brightness control apparatus, however, the picture displayed at the center portion of the screen becomes brighter than that at its corner portions due to the difference in deflection angle between the center and the corner portions. Specifically, since the distance between an electron gun of the CRT 2 and each corner portion of the screen is relatively longer than that between the electron gun and the center portion of the screen, the picture displayed at each corner portion becomes relatively darker than that displayed at the center portion. Especially, such a difference in brightness of the screen portions is greatly intensified in a CRT having a large-sized screen and thus this causes the quality of the picture displayed on the CRT to deteriorate.